Magnetic resonance imaging method and magnetic resonance imaging apparatus that compensate for slab distortion by selective slab thickness expansion

ABSTRACT

A magnetic resonance imaging method and imaging device are disclosed. The magnetic resonance imaging method includes dividing the current slab of an imaging region into an initial number of detection sub-slabs, and expanding the encoded thickness of each detection sub-slab according to a predetermined initial expansion factor, subjecting each expanded detection sub-slab to deformation detection using the first fast spin echo sequence, and determining the position of each imaging sub-slab of the current slab and an expansion factor corresponding to each imaging sub-slab, wherein the readout gradient of the first fast spin echo sequence is applied in the direction of the slice selection gradient, expanding the encoded thickness of each imaging sub-slab of the current slab of the imaging region on the basis of the determined position of each imaging sub-slab and the corresponding expansion factor, and performing an imaging scan of each expanded imaging sub-slab using a second fast spin echo sequence.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of Chinese Patent Office applicationNo. 20110454045.2 CN filed Dec. 30, 2011. All of the applications areincorporated by reference herein in their entirety.

TECHNICAL FIELD

A magnetic resonance imaging method and magnetic resonance imagingdevice are provided.

BACKGROUND ART

Magnetic resonance imaging is a technique for performing imaging usingthe phenomenon of magnetic resonance. The principles of magneticresonance imaging mainly include: in an atomic nucleus containing asingle proton, such as the hydrogen nuclei which are present throughoutthe human body, the proton thereof has spin motion and so resembles asmall magnet. The spin axes of these small magnets have no fixedpattern; if an external magnetic field is applied, the small magnetswill realign in accordance with the lines of magnetic force,specifically aligning in a direction parallel to a line of magneticforce of the external magnetic field, or in a direction antiparallel toa line of magnetic force of the external magnetic field. The directionparallel to a line of magnetic force of the external magnetic field iscalled the positive longitudinal axis, while the direction antiparallelto a line of magnetic force of the external magnetic field is called thenegative longitudinal axis; a nucleus has only a longitudinal componentof magnetization, this longitudinal component of magnetization havingboth a direction and a magnitude. Nuclei in the external magnetic fieldare excited using a Radio Frequency (RF) pulse of a specific frequency,such that the spin axes of these nuclei deviate from the positivelongitudinal axis or negative longitudinal axis, resulting in resonance;this is the phenomenon of magnetic resonance. Once the spin axes of theexcited nuclei have deviated from the positive longitudinal axis ornegative longitudinal axis, the nuclei have a transverse component ofmagnetization.

Once emission of the RF pulse has stopped, the excited nuclei emit anecho signal, gradually releasing the absorbed energy in the form of anelectromagnetic wave, and the phase and energy level thereof return totheir pre-excitation states; by subjecting the echo signals emitted bythe nuclei to further processing, such as spatial encoding, an image canbe reconstructed.

FIGS. 1a to 1c show schematic diagrams of a type of multi-slab imageencoding based on a 3D fast spin echo sequence. FIG. 1a is a schematicdiagram showing partitioning of sub-slabs; FIG. 1b is a schematicdiagram showing partial encoding of the sub-slabs; FIG. 1c is aschematic diagram showing the relationship between the excited layerthickness and expanded layer thickness for each sub-slab.

As FIG. 1a shows, each slab of the imaging region is first divided intomultiple sub-slabs in the slice direction; the case of 8 sub-slabs istaken as an example in FIG. 1 a. An imaging scan is then performed oneach sub-slab using 3D fast spin echoes.

One slice encoding step during specific encoding and imaging is shown inFIG. 1 b, in which RF, SL (Slice), PE (Phase Encoding), RO (Readout) andADC (analog-digital converter) correspond to radio frequency pulse,slice selection gradient, encoding gradient, readout gradient and dataacquisition module, respectively. The method comprises: emitting asequence of pulses of different angles within a repetition time TR, atthe same time varying the phase encoding gradient with a certain sliceencoding gradient, so as to fill one slice encoded k-space; withinanother repetition time TR, the RF pulses remain unchanged and the sliceencoding gradient is changed, giving another slice encoded k-space; andso on until data for the whole k-space is collected. In the pulsesequence, one 90 degree selective exciting pulse is applied first, aslice selection gradient corresponding to the current sub-slab beingapplied in the SL direction at the same time. One al-degree selectiveinverting RF pulse is then emitted; at the same time, a slice selectiongradient corresponding to the current sub-slab and a slice encodinggradient are applied in the SL direction, a first encoding gradient isapplied in the PE direction, and the ADC is then used to perform dataacquisition. One a2-degree selective inverting RF pulse is then emitted;at the same time, a slice selection gradient corresponding to thecurrent sub-slab and a slice encoding gradient are applied in the SLdirection, a second encoding gradient is applied in the PE direction,and the ADC is then used to perform data acquisition, and so on untildata for the whole k-space is collected.

During the imaging process, expansion must be performed in accordancewith a predetermined expansion factor on either side of the sub-slabslice direction, so as to obtain a slice encoded thickness greater thanthe excited thickness, encoding being performed on the slice thicknesscorresponding to this encoded thickness. As shown in FIG. 1 c, thethickness TH corresponding to the middle shaded region of FIG. 1c is theexcited thickness of the current sub-slab, while the thickness STHcorresponding to the whole region of FIG. 1c is the slice encodedthickness of the current sub-slab. The excited thickness is generallyequal to the thickness of the corresponding sub-slab. The expansionfactors corresponding to each sub-slab are generally equal.

Since a metal implant (MI) may be implanted inside a living body for thepurpose of securing or replacing a joint or other vital tissue duringorthopedic surgery and other emergency operations, in practicalapplications the presence of a metal insert will give rise toinhomogeneity in the external magnetic field, leading to geometricdistortion of the image. For each sub-slab, this geometric distortion ismainly embodied in slice deformation of the excited sub-slab, the slicedeformation corresponding to an excited sub-slab being different fordifferent distances between the excited sub-slab and the metal implant.In the schematic diagram of FIG. 2, showing the positions of sub-slabsrelative to a metal implant MI, the slice deformation of the n^(th)sub-slab, which is closer to the metal implant MI, is greater than thatof the m^(th) sub-slab, which is remote from the metal implant MI.

Furthermore, different types of metal implant give rise to differentslice deformations in an excited sub-slab. If the same expansion factoris used to expand the encoded thickness for each sub-slab, fullacquisition of the image data arising from the slice deformation of eachexcited sub-slab is not possible, so the distorted image cannot berestored fully during the image reconstruction stage.

SUMMARY OF INVENTION

In view of the above, a magnetic resonance imaging method and a magneticresonance imaging device are proposed, for fully acquiring and restoringa distorted image, to further improve image quality.

A magnetic resonance imaging method provided in accordance with anembodiment comprises:

dividing the current slab of an imaging region into an initial number ofdetection sub-slabs, and expanding the encoded thickness of eachdetection sub-slab according to a predetermined initial expansionfactor;

subjecting each expanded detection sub-slab to deformation detectionusing a first fast spin echo sequence, and determining the position ofeach imaging sub-slab of the current slab and an expansion factorcorresponding to each imaging sub-slab on the basis of a deformationdetection result, wherein the readout gradient of the first fast spinecho sequence is applied in the direction of the slice selectiongradient;

expanding the encoded thickness of each imaging sub-slab of the currentslab of the imaging region on the basis of the determined position ofeach imaging sub-slab and the corresponding expansion factor;

performing an imaging scan of each expanded imaging sub-slab using asecond fast spin echo sequence.

Optionally, the step of subjecting each expanded detection sub-slab todeformation detection using a first fast spin echo sequence, anddetermining the position of each imaging sub-slab of the current slaband an expansion factor corresponding to each imaging sub-slab on thebasis of a deformation detection result comprises:

subjecting the current expanded detection sub-slab to deformationdetection using a first fast spin echo sequence, to obtain a slicedeformation Δ;

based on the relationships

Δ=Δ1−Δ2,

${\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = \frac{\Delta 1}{TH}}},$obtaining the expansion factor

$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$

corresponding to the detection sub-slab, wherein Δ1 is the excitationdeformation, Δ2 is the readout gradient deformation, GS_(ro) is theamplitude of the readout gradient applied in the direction of the sliceselection gradient, GS_(SS) is the amplitude of the slice selectiongradient, a is a coefficient determined on the basis of the excitedthickness, the size of the imaging region covered by the applied readoutgradient, and the size of the resolution of the readout gradient in thedirection of the slice selection gradient, and TH is the excitedthickness;

taking the position corresponding to each detection sub-slab of thecurrent slab as the position of each imaging sub-slab of the currentslab, and taking the expansion factor obtained for each detectionsub-slab as the expansion factor of the imaging sub-slab correspondingthereto.

Optionally, the step of subjecting each expanded detection sub-slab todeformation detection using a first fast spin echo sequence, anddetermining the position of each imaging sub-slab of the current slaband an expansion factor corresponding to each imaging sub-slab on thebasis of a deformation detection result comprises:

A. subjecting the current expanded detection sub-slab to deformationdetection using a first fast spin echo sequence, to obtain a slicedeformation Δ;

B. based on the relationships

Δ=Δ1−Δ2,

${\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = \frac{\Delta 1}{TH}}},$obtaining the expansion factor

$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$

corresponding to the detection sub-slab, wherein Δ1 is the excitationdeformation, Δ2 is the readout gradient deformation, GS_(ro) is theamplitude of the readout gradient applied in the direction of the sliceselection gradient, GS_(SS) is the amplitude of the slice selectiongradient, a is a coefficient determined on the basis of the excitedthickness, the size of the imaging region covered by the applied readoutgradient, and the size of the resolution of the readout gradient in thedirection of the slice selection gradient, and TH is the excitedthickness;

C. keeping the encoded thickness of each detection sub-slab the same,adjusting the excited thickness of each detection sub-slab according tothe obtained expansion factor corresponding to each detection sub-slab,and adjusting the current number of detection sub-slabs of the currentslab and the position corresponding to each detection sub-slab accordingto the adjusted excited thickness of each detection sub-slab;

D. if the current number is equal to the initial number, taking thenewly determined position corresponding to each detection sub-slab ofthe current slab as the position of each imaging sub-slab of the currentslab, and taking the expansion factor obtained for each detectionsub-slab as the expansion factor of the imaging sub-slab correspondingthereto; otherwise, taking the current number as an initial number,dividing the current slab into an initial number of detection sub-slabsaccording to the newly determined positions corresponding to eachdetection sub-slab of the current slab, expanding the encoded thicknessof each newly added detection sub-slab according to a predeterminedinitial expansion factor, expanding the encoded thickness of existingdetection sub-slabs after adjustment according to each obtainedexpansion factor, and returning to step A.

Optionally, the step of expanding the encoded thickness of each imagingsub-slab of the current slab of the imaging region on the basis of thedetermined position of each imaging sub-slab and the correspondingexpansion factor comprises:

expanding each imaging sub-slab symmetrically on either side of theslice selection direction of the imaging sub-slab according to theexpansion factor corresponding to the imaging sub-slab; or

expanding each imaging sub-slab asymmetrically on either side of theslice selection direction of the imaging sub-slab according to theexpansion factor corresponding to the imaging sub-slab and an excitationdeformation direction corresponding to the expansion factor.

Optionally, the first fast spin echo sequence is a one-dimensional or atwo-dimensional fast spin echo sequence.

Optionally, the second fast spin echo sequence is a two-dimensional or athree-dimensional fast spin echo sequence.

A magnetic resonance imaging device provided in accordance with anotherembodiment comprises:

a deformation detection module, for dividing the current slab of animaging region into an initial number of detection sub-slabs, expandingthe encoded thickness of each detection sub-slab according to apredetermined initial expansion factor, subjecting each expandeddetection sub-slab to deformation detection using a first fast spin echosequence, and determining the position of each imaging sub-slab of thecurrent slab and an expansion factor corresponding to each imagingsub-slab on the basis of a deformation detection result, wherein thereadout gradient of the first fast spin echo sequence is applied in thedirection of the slice selection gradient;

an imaging scan module, for expanding the encoded thickness of eachimaging sub-slab of the current slab of the imaging region on the basisof the position of each imaging sub-slab and the expansion factorcorresponding to each imaging sub-slab as determined by the deformationdetection module, and performing an imaging scan of each expandedimaging sub-slab using a second fast spin echo sequence.

Optionally, the deformation detection module comprises:

a first sub-slab partitioning sub-module, for dividing the current slabof an imaging region into an initial number of detection sub-slabs, andexpanding the encoded thickness of each detection sub-slab according toa predetermined initial expansion factor;

a first slice deformation detection sub-module, for subjecting eachexpanded detection sub-slab to deformation detection using a first fastspin echo sequence, to obtain a slice deformation Δ;

a first expansion factor calculation sub-module, for obtaining theexpansion factor

$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$corresponding to the detection sub-slab on the basis of therelationships Δ=Δ1−Δ2,

${\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = \frac{\Delta 1}{TH}}},$

wherein Δ1 is the excitation deformation, Δ2 is the readout gradientdeformation, GS_(ro) is the amplitude of the readout gradient applied inthe direction of the slice selection gradient, GS_(SS) is the amplitudeof the slice selection gradient, a is a coefficient determined on thebasis of the excited thickness, the size of the imaging region coveredby the applied readout gradient, and the size of the resolution of thereadout gradient in the direction of the slice selection gradient, andTH is the excited thickness;

a first result determination sub-module, for taking the positioncorresponding to each detection sub-slab of the current slab as theposition of each imaging sub-slab of the current slab, and taking theexpansion factor obtained for each detection sub-slab as the expansionfactor of the imaging sub-slab corresponding thereto.

Optionally, the deformation detection module comprises:

a second sub-slab partitioning sub-module, for dividing the current slabof an imaging region into an initial number of detection sub-slabs, andexpanding the encoded thickness of each detection sub-slab according toa predetermined initial expansion factor;

a second slice deformation detection sub-module, for subjecting eachexpanded detection sub-slab to deformation detection using a first fastspin echo sequence, to obtain a slice deformation Δ;

a second expansion factor calculation sub-module, for obtaining theexpansion factor

$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$corresponding to the detection sub-slab on the basis of therelationships Δ=Δ1−Δ2,

${\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = \frac{\Delta 1}{TH}}},$

wherein Δ1 is the excitation deformation, Δ2 is the readout gradientdeformation, GS_(ro) is the amplitude of the readout gradient applied inthe direction of the slice selection gradient, GS_(SS) is the amplitudeof the slice selection gradient, a is a coefficient determined on thebasis of the excited thickness, the size of the imaging region coveredby the applied readout gradient, and the size of the resolution of thereadout gradient in the direction of the slice selection gradient, andTH is the excited thickness;

a sub-slab adjustment sub-module, for keeping the encoded thickness ofeach detection sub-slab the same, adjusting the excited thickness ofeach detection sub-slab according to the obtained expansion factorcorresponding to each detection sub-slab, and adjusting the currentnumber of detection sub-slabs of the current slab and the positioncorresponding to each detection sub-slab according to the adjustedexcited thickness of each detection sub-slab;

a second result determination sub-module, for taking the newlydetermined position corresponding to each detection sub-slab of thecurrent slab as the position of each imaging sub-slab of the currentslab, and taking the expansion factor obtained for each detectionsub-slab as the expansion factor of the imaging sub-slab correspondingthereto, when the current number is equal to the initial number;otherwise, taking the current number as an initial number, dividing thecurrent slab of the imaging region into the current number of detectionsub-slabs according to the newly determined positions corresponding toeach detection sub-slab of the current slab, expanding the encodedthickness of each newly added detection sub-slab according to apredetermined initial expansion factor, expanding the encoded thicknessof existing detection sub-slabs after adjustment according to eachobtained expansion factor, and advising the slice deformation detectionmodule to subject the currently partitioned detection sub-slabs to slicedetection.

Optionally, the imaging scan module comprises: a first expansionsub-module and an imaging scan sub-module; alternatively, it comprises:a second expansion sub-module and an imaging scan sub-module;

the first expansion sub-module is used for expanding each imagingsub-slab symmetrically on either side of the slice selection directionof the imaging sub-slab according to the expansion factor correspondingto the imaging sub-slab;

the second expansion sub-module is used for expanding each imagingsub-slab asymmetrically on either side of the slice selection directionof the imaging sub-slab according to the expansion factor correspondingto the imaging sub-slab and an excitation deformation directioncorresponding to the expansion factor;

the imaging scan sub-module is used for performing an imaging scan ofeach expanded imaging sub-slab using a second fast spin echo sequence.

It can be seen from the above solution that in the embodiments, fullacquisition and restoration of a distorted image is possible, becausethe current slab of the imaging region is divided into a plurality ofdetection sub-slabs before being subjected to an imaging scan, eachdetection sub-slab is then subjected to deformation detection so as tofinally obtain the expansion factors corresponding to each encodedsub-slab of the current slab, and the expansion factors corresponding toeach encoded sub-slab are then used to subject each encoded sub-slab toencoded thickness expansion and an imaging scan in a targeted way.

Furthermore, when the encoded thickness of each imaging sub-slab isexpanded using the expansion factor corresponding thereto, asymmetricalexpansion can be performed on either side of the slice selectiondirection of the imaging sub-slab according to an excitation deformationdirection corresponding to the deformation detection time, therebyensuring that the distorted image can be fully acquired and restored,and further improving image quality.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described in detail below with reference to theaccompanying drawings, so as to furnish those skilled in the art with aclearer understanding of the above and other features and advantages; inthe drawings:

FIGS. 1a to 1c are schematic diagrams of a type of multi-slab imageencoding based on a 3D fast spin echo sequence. FIG. 1a is a schematicdiagram showing partitioning of sub-slabs; FIG. 1b is a schematicdiagram showing partial encoding of the sub-slabs; FIG. 1c is aschematic diagram showing the relationship between the excited layerthickness and expanded layer thickness for each sub-slab.

FIG. 2 is a schematic diagram showing the positions of sub-slabsrelative to a metal implant.

FIG. 3 is an illustrative flow chart of the magnetic resonance imagingmethod in the embodiments.

FIG. 4 is a schematic diagram of one slice encoding step duringdeformation detection in the embodiments.

FIGS. 5a to 5c are schematic diagrams of slice deformation and theexcitation deformation and readout gradient deformation correspondingthereto in the embodiments. FIG. 5a is a schematic diagram of slicedeformation; FIG. 5b is a schematic diagram of the excitationdeformation; FIG. 5c is a schematic diagram of the readout gradientdeformation.

FIGS. 6a and 6b are schematic diagrams showing the encoded thicknesscorresponding to different sub-slabs when the excited thickness is keptunchanged in the embodiments. FIG. 6a is a schematic diagram showing theencoded thickness corresponding to a sub-slab that is closer to a metalimplant; FIG. 6b is a schematic diagram showing the encoded thicknesscorresponding to a sub-slab that is further away from the metal implant.

FIGS. 7a and 7b are schematic diagrams showing the excited thicknesscorresponding to different sub-slabs when the encoded thickness is keptunchanged in the embodiments. FIG. 7a is a schematic diagram showing theexcited thickness corresponding to a sub-slab that is close to a metalimplant; FIG. 7b is a schematic diagram showing the excited thicknesscorresponding to a sub-slab that is remote from the metal implant.

FIGS. 8a and 8b are schematic diagrams showing a method for expandingthe encoded thickness of a sub-slab using an expansion factor in theembodiments. FIG. 8a is a schematic diagram showing a sub-slab beingsubjected to symmetric expansion; FIG. 8b is a schematic diagram showinga sub-slab being subjected to asymmetric expansion.

FIG. 9 is an illustrative structural diagram of a magnetic resonanceimaging device in the embodiments.

FIG. 10 is a structural schematic diagram of a deformation detectionmodule in the embodiments.

FIG. 11 is another structural schematic diagram of a deformationdetection module in the embodiments.

FIG. 12 is a structural schematic diagram of an imaging scan module inthe embodiments.

FIG. 13 is another structural schematic diagram of an imaging scanmodule in the embodiments.

In the drawings, the reference labels are as follows:

-   SD—slice direction TH—excited thickness STH—encoded thickness-   RF—radio frequency pulse SL—slice selection gradient-   PE—encoding gradient RO—readout gradient-   ADC—data acquisition module-   m—m^(th) sub-slab n—n^(th) sub-slab-   DD—deformation direction Δ—slice deformation-   Δ1—excitation deformation-   Δ2—readout gradient deformation-   301—sub-slab partitioning 302—deformation detection-   303—encoding expansion 304—imaging scan-   901—deformation detection module-   902—imaging scan module-   1001—first sub-slab partitioning sub-module-   1002—first slice deformation detection sub-module-   1003—first expansion factor calculation sub-module-   1004—first result determination sub-module-   1101—second sub-slab partitioning sub-module-   1102—second slice deformation detection sub-module-   1103—second expansion factor calculation sub-module-   1104—sub-slab adjustment sub-module-   1105—second result determination module-   1201—first expansion sub-module-   1202—first imaging scan sub-module-   1301—second expansion sub-module-   1302—second imaging scan sub-module

In view of the fact that the slice deformation corresponding to anexcited sub-slab varies with the distance of the excited sub-slab from ametal implant, and the fact that different types of metal implant causedifferent slice deformations of excited sub-slabs, in order to fullyacquire and restore a distorted image in the embodiments, each sub-slabof the current slab of the imaging region is subjected to deformationdetection before an imaging scan of the current sub-slab is performed,so as to obtain a slice deformation value corresponding to eachsub-slab; a suitable expansion factor corresponding to the sub-slab isthen obtained based on the slice deformation value, and encodedthickness expansion is then performed in a targeted way using theexpansion factor corresponding to each sub-slab.

The embodiments are described in further detail below by way ofexamples, in order to clarify the object, technical solution andadvantages thereof.

FIG. 3 is an illustrative flow chart of the magnetic resonance imagingmethod in the embodiments. As FIG. 3 shows, the method comprises thefollowing steps:

Step 301, dividing the current slab of an imaging region into an initialnumber of detection sub-slabs, and expanding the encoded thickness ofeach detection sub-slab according to a predetermined initial expansionfactor.

The specific implementation process of this step can be the same as theprocess of sub-slab partitioning and encoding thickness expansion duringan imaging scan in the prior art. Alternatively, the initial number andinitial expansion factor in this step may also be determined on thebasis of empirical values or simulated values, etc.

Step 302, subjecting each expanded detection sub-slab to deformationdetection using a first fast spin echo sequence, and determining theposition of each imaging sub-slab of the current slab and an expansionfactor corresponding to each imaging sub-slab on the basis of adeformation detection result.

In this embodiment, the readout gradient of the first fast spin echosequence is applied in the direction of the slice selection gradientrather than in the readout gradient direction. This is shown in FIG. 4,which shows a schematic diagram of one slice encoding step duringdeformation detection in this embodiment. As can be seen, in this pulsesequence a 90-degree selective excitation pulse is applied first, aslice selection gradient corresponding to the current sub-slab and aphase pre-dispersion gradient being applied in the SL direction at thesame time. An al-degree selective inverting RF pulse is then emitted; atthe same time, a slice selection gradient corresponding to the currentsub-slab and a readout gradient are applied in the SL direction, a firstencoding gradient is applied in the PE direction, and the ADC is thenused to perform data acquisition. An a2-degree selective inverting RFpulse is then emitted; at the same time, a slice selection gradientcorresponding to the current sub-slab and a readout gradient are appliedin the SL direction, a second encoding gradient is applied in the PEdirection, and the ADC is then used to perform data acquisition, and soon until data for the whole k-space is collected. As can be seen, inthis sequence the readout gradient is moved from the RO direction to theSL direction, so the sequence is a two-dimensional fast spin echosequence. During practical application, a one-dimensional fast spin echosequence may also be used to subject each sub-slab to deformationdetection; for instance, the sequence shown in FIG. 4 may be turned intoa one-dimensional fast spin echo sequence simply by removing theencoding gradient in the PE direction.

During particular implementation, many particular embodiments of thisstep are possible, two of which are given below:

First embodiment: the encoded thickness is adjusted on the basis of theresult of deformation detection. In particular, this may comprise:

1) Subjecting the current expanded detection sub-slab to deformationdetection using a first fast spin echo sequence, to obtain a slicedeformation Δ.

In step 1), when the current expanded detection sub-slab is subjected todeformation detection using a first fast spin echo sequence, the slicedeformation Δ may be measured, as shown in FIG. 5a . FIGS. 5a to 5c areschematic diagrams of slice deformation and the excitation deformationand readout gradient deformation corresponding thereto in theembodiments. In fact, the slice deformation Δ includes deformationinformation in two parts, these being the excitation deformation Δ1shown in FIG. 5b and the readout gradient deformation Δ2 shown in FIG.5c , the three quantities satisfying the relation Δ=Δ1−Δ2 . Theexcitation deformation Δ1 and the readout gradient deformation Δ2 havedifferent deformation directions and in general cannot be obtaineddirectly.

2) Based on the relationships Δ=Δ1−Δ2,

${\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = \frac{\Delta 1}{TH}}},$obtaining the expansion factor

$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$

corresponding to the detection sub-slab, wherein GS_(ro) is theamplitude of the readout gradient applied in the direction of the sliceselection gradient, GS_(SS) i s the amplitude of the slice selectiongradient, a is a coefficient determined on the basis of the excitedthickness, the size of the imaging region covered by the applied readoutgradient, and the size of the resolution of the readout gradient in thedirection of the slice selection gradient, TH is the excited thickness,and f is the expansion factor.

The amplitude GS_(ro) of the readout gradient applied in the directionof the slice selection gradient, the amplitude GS_(SS) of the sliceselection gradient, the coefficient a and the excited thickness TH canall be obtained in advance.

3) Taking the position corresponding to each detection sub-slab of thecurrent slab as the position of each imaging sub-slab of the currentslab, and taking the expansion factor obtained for each detectionsub-slab as the expansion factor of the imaging sub-slab correspondingthereto.

In this particular embodiment, the main point is that the number ofsub-slabs and the excited thicknesses thereof are kept the same, so thatthe partitioning of sub-slabs of the current slab during the imagingscan is the same as the partitioning of sub-slabs during deformationdetection; however, when the encoded thickness of each encoded sub-slabis expanded in step 303, expansion is performed separately according tothe different expansion factors corresponding to each encoding sub-slab,so that the encoded thickness of each sub-slab during the imaging scanis different from the encoded thickness of each sub-slab duringdeformation detection. FIGS. 6a and 6b are schematic diagrams showingthe encoded thickness corresponding to different sub-slabs when theexcited thickness is kept unchanged in the embodiments. As FIG. 6ashows, since the slice deformation is greater for the n^(th) sub-slabthat is closer to the metal implant MI, the corresponding expansionfactor is greater; when the encoded thickness of this sub-slab isexpanded using this expansion factor, a greater encoded thickness isobtained. As FIG. 6b shows, since the slice deformation is smaller forthe m^(th) sub-slab that is further away from the metal implant MI, thecorresponding expansion factor is smaller; when the encoded thickness ofthis sub-slab is expanded using this expansion factor, a smaller encodedthickness is obtained.

Second embodiment: the excited thickness is adjusted on the basis of theresult of deformation detection. In particular, this may comprise:

A. Subjecting the current expanded detection sub-slab to deformationdetection using a first fast spin echo sequence, to obtain a slicedeformation Δ.

B. Based on the relationships Δ=Δ1−Δ2,

${\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = \frac{\Delta 1}{TH}}},$obtaining the expansion factor

$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$

corresponding to the detection sub-slab, wherein Δ1 is the excitationdeformation, Δ2 is the readout gradient deformation, GS_(ro) is theamplitude of the readout gradient applied in the direction of the sliceselection gradient, GS_(SS) is the amplitude of the slice selectiongradient, a is a coefficient determined on the basis of the excitedthickness, the size of the imaging region covered by the applied readoutgradient, and the size of the resolution of the readout gradient in thedirection of the slice selection gradient, and TH is the excitedthickness.

C. Keeping the encoded thickness of each detection sub-slab the same,adjusting the excited thickness of each detection sub-slab according tothe obtained expansion factor corresponding to each detection sub-slab,and adjusting the current number of detection sub-slabs of the currentslab and the position corresponding to each detection sub-slab accordingto the adjusted excited thickness of each detection sub-slab.

During practical application, the encoded thickness typicallycorresponds to a certain slice encoding step, so that keeping theencoded thickness of each detection sub-slab the same is equivalent tokeeping the total number of encoding steps of each detection sub-slabthe same, in which case the adjusted excited thickness TH_(j) can bedetermined according to the relation TH_(j)=N_(j)/f_(j) in order tofully acquire and restore the distorted image. N_(j) is the total numberof encoding steps of the j^(th) detection sub-slab, f_(j) is theexpansion factor corresponding to the j^(th) detection sub-slab, andTH_(j) is the adjusted excited thickness of the j^(th) detectionsub-slab. FIGS. 7a and 7b are schematic diagrams showing the excitedthickness corresponding to different sub-slabs when the encodedthickness is kept unchanged in the embodiments. As FIG. 7a shows, sincethe slice deformation is greater for the n^(th) sub-slab that is closerto the metal implant MI, the corresponding expansion factor is greater;when the encoded thickness is kept the same and the excited thickness ofthe sub-slab is adjusted using this expansion factor, a smaller excitedthickness is obtained. As FIG. 7b shows, since the slice deformation issmaller for the m^(th) sub-slab that is further away from the metalimplant MI, the corresponding expansion factor is smaller; when theencoded thickness is kept the same and the excited thickness of thesub-slab is adjusted using this expansion factor, a greater excitedthickness is obtained.

Since the excited thickness is equal to the thickness of the detectionsub-slab, once the excited thickness is adjusted, the thickness andposition of the detection sub-slab will change accordingly. In this way,the partitioning of sub-slabs in the current slab will change, and it ispossible that new detection sub-slabs will need to be added;correspondingly, the number of detection sub-slabs may also change.

D. Judging whether the current number is equal to the initial number. Ifit is, step E is performed; otherwise, step F is performed.

If the current number is equal to the initial number, then the number ofexpansion factors is equal to the current number of adjusted sub-slabs,in which case step E may be performed; otherwise, the number ofexpansion factors is different from the current number of adjustedsub-slabs, and it is necessary to perform deformation detection again tomake the two numbers equal.

E. Taking the newly determined position corresponding to each detectionsub-slab of the current slab as the position of each imaging sub-slab ofthe current slab, and taking the expansion factor obtained for eachdetection sub-slab as the expansion factor of the imaging sub-slabcorresponding thereto.

F. Taking the current number as an initial number, dividing the currentslab into an initial number of detection sub-slabs according to thenewly determined positions corresponding to each detection sub-slab ofthe current slab, expanding the encoded thickness of each newly addeddetection sub-slab according to a predetermined initial expansionfactor, expanding the encoded thickness of existing detection sub-slabsafter adjustment according to each obtained expansion factor, andreturning to step A.

Step 303, expanding the encoded thickness of each imaging sub-slab ofthe current slab of the imaging region on the basis of the determinedposition of each imaging sub-slab and the corresponding expansionfactor.

In this step, for each imaging sub-slab, the expansion factorcorresponding thereto is used to expand the encoded thickness thereof.Since different slice deformations correspond to different expansionfactors, each sub-slab can be expanded to a different encoded thicknessaccording to actual requirements, so as to fully acquire and restore thedistorted image.

During practical application, when the expansion factor corresponding toeach imaging sub-slab is used to expand the encoded thickness thereof,symmetric or asymmetric expansion may be performed on either side of theslice selection direction of the imaging sub-slab. For instance, basedon the direction of excitation deformation corresponding to eachexpansion factor during deformation detection, greater expansion may beperformed on the excitation deformation direction side, and lesserexpansion on the other side; the specific ratio of expansions on the twosides can be determined on the basis of the specific result ofdeformation detection. FIGS. 8a and 8b are schematic diagrams showing amethod for expanding the encoded thickness of a sub-slab using anexpansion factor in the embodiments. FIG. 8a is a schematic diagramshowing a sub-slab being subjected to symmetric expansion; FIG. 8b is aschematic diagram showing a sub-slab being subjected to asymmetricexpansion.

Step 304, performing an imaging scan of each expanded imaging sub-slabusing a second fast spin echo sequence.

In this step, the second fast spin echo sequence can be atwo-dimensional fast spin echo sequence or a three-dimensional fast spinecho sequence.

FIG. 9 is an illustrative structural diagram of a magnetic resonanceimaging device in the embodiments. As FIG. 9 shows, the magneticresonance imaging device comprises: a deformation detection module 901and an imaging scan module 902.

The deformation detection module 901 is used for dividing the currentslab of an imaging region into an initial number of detection sub-slabs,expanding the encoded thickness of each detection sub-slab according toa predetermined initial expansion factor, subjecting each expandeddetection sub-slab to deformation detection using a first fast spin echosequence, and determining the position of each imaging sub-slab of thecurrent slab and an expansion factor corresponding to each imagingsub-slab on the basis of a deformation detection result. In thisembodiment, the readout gradient of the first fast spin echo sequence isapplied in the direction of the slice selection gradient rather than inthe readout gradient direction.

The imaging scan module 902 is used for expanding the encoded thicknessof each imaging sub-slab of the current slab of the imaging region onthe basis of the position of each imaging sub-slab and the expansionfactor corresponding to each imaging sub-slab as determined by thedeformation detection module, and performing an imaging scan of eachexpanded imaging sub-slab using a second fast spin echo sequence.

During specific implementation, many particular embodiments of thedeformation detection module 901 are possible. Only two such embodimentsare given below.

FIG. 10 is a structural schematic diagram of a deformation detectionmodule in this embodiment. As FIG. 10 shows, the deformation detectionmodule comprises: a first sub-slab partitioning sub-module 1001, a firstslice deformation detection sub-module 1002, a first expansion factorcalculation sub-module 1003 and a first result determination sub-module1004.

The first sub-slab partitioning sub-module 1001 is used for dividing thecurrent slab of an imaging region into an initial number of detectionsub-slabs, and expanding the encoded thickness of each detectionsub-slab according to a predetermined initial expansion factor.

The first slice deformation detection sub-module 1002 is used forsubjecting each expanded detection sub-slab to deformation detectionusing a first fast spin echo sequence, to obtain a slice deformation Δ.

The first expansion factor calculation sub-module 1003 is used forobtaining the expansion factor

$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$corresponding to the detection sub-slab on the basis of therelationships Δ=Δ1−Δ2,

$\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = {\frac{\Delta 1}{TH}.}}$

Here, Δ1 is the excitation deformation, Δ2 is the readout gradientdeformation, GS_(ro) is the amplitude of the readout gradient applied inthe direction of the slice selection gradient, GS_(SS) is the amplitudeof the slice selection gradient, a is a coefficient determined on thebasis of the excited thickness, the size of the imaging region coveredby the applied readout gradient, and the size of the resolution of thereadout gradient in the direction of the slice selection gradient, andTH is the excited thickness.

The first result determination sub-module 1004 is used for taking theposition corresponding to each detection sub-slab of the current slab asthe position of each imaging sub-slab of the current slab, and takingthe expansion factor obtained for each detection sub-slab as theexpansion factor of the imaging sub-slab corresponding thereto.

FIG. 11 is another structural schematic diagram of a deformationdetection module in this embodiment. As FIG. 11 shows, the deformationdetection module comprises: a second sub-slab partitioning sub-module1101, a second slice deformation detection sub-module 1102, a secondexpansion factor calculation sub-module 1103, a sub-slab adjustmentsub-module 1104 and a second result determination sub-module 1105.

The second sub-slab partitioning sub-module 1101 is used for dividingthe current slab of an imaging region into an initial number ofdetection sub-slabs, and expanding the encoded thickness of eachdetection sub-slab according to a predetermined initial expansionfactor.

The second slice deformation detection sub-module 1102 is used forsubjecting each expanded detection sub-slab to deformation detectionusing a first fast spin echo sequence, to obtain a slice deformation Δ.

The second expansion factor calculation sub-module 1103 is used forobtaining the expansion factor

$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$corresponding to the detection sub-slab on

the basis of the relationships Δ=Δ1−Δ2,

${\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = \frac{\Delta 1}{TH}}},$

Here, Δ1 is the excitation deformation, Δ2 is the readout gradientdeformation, GS_(ro) is the amplitude of the readout gradient applied inthe direction of the slice selection gradient, GS_(SS) is the amplitudeof the slice selection gradient, a is a coefficient determined on thebasis of the excited thickness, the size of the imaging region coveredby the applied readout gradient, and the size of the resolution of thereadout gradient in the direction of the slice selection gradient, andTH is the excited thickness.

The sub-slab adjustment sub-module 1104 is used for keeping the encodedthickness of each detection sub-slab the same, adjusting the excitedthickness of each detection sub-slab according to the obtained expansionfactor corresponding to each detection sub-slab, and adjusting thecurrent number of detection sub-slabs of the current slab and theposition corresponding to each detection sub-slab according to theadjusted excited thickness of each detection sub-slab.

The second result determination sub-module 1105 is used for taking thenewly determined position corresponding to each detection sub-slab ofthe current slab as the position of each imaging sub-slab of the currentslab, and taking the expansion factor obtained for each detectionsub-slab as the expansion factor of the imaging sub-slab correspondingthereto, when the current number is equal to the initial number;otherwise, taking the current number as an initial number, dividing thecurrent slab of the imaging region into the current number of detectionsub-slabs according to the newly determined positions corresponding toeach detection sub-slab of the current slab, expanding the encodedthickness of each newly added detection sub-slab according to apredetermined initial expansion factor, expanding the encoded thicknessof existing detection sub-slabs after adjustment according to eachobtained expansion factor, and advising the slice deformation detectionmodule to subject the currently partitioned detection sub-slabs to slicedetection.

During specific implementation, many particular embodiments of theimaging scan module 902 are possible. Only two such embodiments aregiven below.

FIG. 12 is a structural schematic diagram of an imaging scan module inthis embodiment. As FIG. 12 shows, the imaging scan module comprises: afirst expansion sub-module 1201 and a first imaging scan sub-module1202.

The first expansion sub-module 1201 is used for expanding each imagingsub-slab symmetrically on either side of the slice selection directionof the imaging sub-slab according to the expansion factor correspondingto the imaging sub-slab.

The first imaging scan sub-module 1202 is used for performing an imagingscan of each expanded imaging sub-slab using a second fast spin echosequence.

FIG. 13 is another structural schematic diagram of an imaging scanmodule in this embodiment. As FIG. 13 shows, the imaging scan modulecomprises: a second expansion sub-module 1301 and a second imaging scansub-module 1302.

The second expansion sub-module 1301 is used for expanding each imagingsub-slab asymmetrically on either side of the slice selection directionof the imaging sub-slab according to the expansion factor correspondingto the imaging sub-slab and an excitation deformation directioncorresponding to the expansion factor.

The second imaging scan sub-module 1302 is used for performing animaging scan of each expanded imaging sub-slab using a second fast spinecho sequence.

Those skilled in the art should appreciate that each accompanyingdrawing is merely a schematic diagram of a preferred embodiment, andthat modules or procedures in the drawings are not necessarily requiredfor implementation.

Those skilled in the art should appreciate that the modules in thedevice in an embodiment may be distributed in the device of theembodiment according to the description of the embodiment, or be changedaccordingly and located in one or more devices different from thisembodiment. Modules in the above embodiments may be combined to form onemodule, or be split further to form multiple sub-modules.

Some of the steps in the embodiments may be implemented using software,with corresponding software programs stored in a readable storage mediumsuch as a CD or hard disk.

A magnetic resonance imaging method and magnetic resonance imagingdevice are disclosed. The magnetic resonance imaging method comprises:dividing the current slab of an imaging region into an initial number ofdetection sub-slabs, and expanding the encoded thickness of eachdetection sub-slab according to a predetermined initial expansionfactor; subjecting each expanded detection sub-slab to deformationdetection using a first fast spin echo sequence, and determining theposition of each imaging sub-slab of the current slab and an expansionfactor corresponding to each imaging sub-slab, wherein the readoutgradient of the first fast spin echo sequence is applied in thedirection of the slice selection gradient; expanding the encodedthickness of each imaging sub-slab of the current slab of the imagingregion on the basis of the determined position of each imaging sub-slaband the corresponding expansion factor; and performing an imaging scanof each expanded imaging sub-slab using a second fast spin echosequence. The embodiments are capable of fully acquiring and restoring adistorted image.

We claim:
 1. A magnetic resonance (MR) imaging method, comprising:providing a computer with a designation of a current slab of anexamination subject from which MR data are to be acquired, said subjectexhibiting a slab-distorting source; utilizing said computer in order todivide said current slab into an initial number of detection sub-slabs,each having thickness, according to a predetermined initial expansionfactor, thereby producing a plurality of expanded detection sub-slabs;utilizing said computer in order to operate an MR data acquisitionscanner comprising a radio-frequency (RF) radiator and a gradient coilarrangement, while the examination subject is in the MR data acquisitionscanner, in order to perform a first fast spin echo sequence in which areadout gradient is activated by said gradient coil arrangement in asame direction as a slice selection gradient, so as to obtain adetection MR dataset from each of the expanded detection sub-slabs, eachof the expanded detection sub-slabs having a sub-slab thicknessrepresented in the detection MR dataset thereof, each sub-slab thicknesshaving two thickness parameters consisting of an excitation thickness,in which nuclear spins in the respective detection sub-slabs wereexcited by said RF radiator, and an encoding thickness, produced fromencoding by said readout gradient; utilizing said computer to determinea position of each expanded detection sub-slab relative to theslab-distorting source of the subject; utilizing said computer to expandeach respective expanded detection sub-slab according to said positionof the respective expanded detection sub-slab relative to saidslab-distorting source of the subject, and said pre-determined initialexpansion factor, by expanding one of said two thickness parameterswhile keeping the other of said two thickness parameters unchanged,thereby producing a plurality of diagnostic sub-slabs; utilizing saidcomputer to operate said MR data acquisition scanner in order to performa second fast spin echo sequence so as to acquire diagnostic MR datafrom each of said diagnostic sub-slabs; and in said computer,reconstructing MR image data for each of said diagnostic sub-slabsrespectively obtained from the acquired diagnostic MR data from each ofsaid diagnostic sub-slabs and emitting the reconstructed MR image datafrom the computer in electronic form in a data file.
 2. The method asclaimed in claim 1 comprising utilizing said computer in order todetermine the position of each expanded detection sub-slab relative tothe slab-distorting source by calculating, for each respective expandeddetection sub-slab, a slice deformation Δ according to Δ=Δ1−Δ2,${\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = \frac{\Delta 1}{TH}}},$in order to obtain an expansion factor F for each respective expandeddetection sub-slab, wherein$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$wherein Δ1 is a deformation resulting from excitation of the respectivesub-slabs, Δ2 is a deformation due to said readout gradient, GS_(ro) isan amplitude of the readout gradient applied in the direction of theslice gradient, GS_(SS) is an amplitude of the slice selection gradient,TH is said sub-slab thickness, and a is a coefficient determined in saidcomputer based on TH, a size of an imaging region covered by the appliedreadout gradient, and a resolution of the readout gradient.
 3. Themethod as claimed in claim 2 comprising: utilizing said computer inorder to set said sub-slab thickness TH of each detection sub-slab,while keeping a total thickness of each detection sub-slab equal,according to the expansion factor for each detection sub-slab, andadjusting a number of said detection sub-slabs, and the respectivepositions thereof, according to the adjusted thickness of each detectionsub-slab; and when a current number of said detection sub-slabs is equalto said initial number, utilizing the position of each detectionsub-slab as the position of each diagnostic sub-slab, and using theexpansion factor of each detection sub-slab as the expansion factor ofeach diagnostic sub-slab, and when said current number is not equal tosaid initial number, using said current number as the initial number anddividing the current slab into said initial number of detectionsub-slabs according to said positions, and expanding a thickness of eachsub-slab according to a predetermined initial expansion factor.
 4. Themethod as claimed in claim 1 comprising utilizing said computer in orderto expand each detection sub-slab symmetrically on opposite sides of thedirection of the slice selection gradient, or in order to expand eachexpanded detection sub-slab asymmetrically on opposite sides of thedirection of the slice selection gradient.
 5. The method as claimed inclaim 1 comprising utilizing said computer to operate said MR dataacquisition scanner in order to perform said first fast spin echosequence as a one-dimensional fast spin echo sequence or atwo-dimensional fast spin echo sequence.
 6. The method as claimed inclaim 1 comprising utilizing said computer to operate said MR dataacquisition scanner in order to perform said second fast spin echosequence as a one-dimensional fast spin echo sequence or atwo-dimensional fast spin echo sequence.
 7. A magnetic resonance (MR)imaging apparatus comprising: an MR data acquisition scanner comprisinga radio-frequency (RF) radiator and a gradient coil arrangement; acomputer provided with a designation of a current slab of an examinationsubject from which MR data are to be acquired, said subject exhibiting aslab-distorting source therein; said computer being configured to dividesaid current slab into an initial number of detection sub-slabs, eachhaving thickness, according to a predetermined initial expansion factor,thereby producing a plurality of expanded detection sub-slabs; saidcomputer being configured to operate an MR data acquisition scanner,while the examination subject is in the MR data acquisition scanner, inorder to perform a first fast spin echo sequence in which a readoutgradient is activated by said gradient coil arrangement in a samedirection as a slice selection gradient, so as to obtain a detection MRdataset from each of the expanded detection sub-slabs, each of theexpanded detection sub-slabs having a sub-slab thickness represented inthe detection MR dataset thereof, each sub-slab thickness having twothickness parameters consisting of an excitation thickness, in whichnuclear spins in the respective detection sub-slabs were excited by saidRF radiator, and an encoding thickness, produced from encoding by saidreadout gradient; said computer being configured to determine a positionof each expanded detection sub-slab relative to the slab-distortingsource of the subject; said computer being configured to expand eachrespective expanded detection sub-slab according to said position of therespective expanded detection sub-slab relative to said slab-distortingsource of the subject, and said pre-determined initial expansion factor,by expanding one of said two thickness parameters while keeping theother of said two thickness parameters unchanged, thereby producing aplurality of diagnostic sub-slabs; said computer being configured tooperate said MR data acquisition scanner in order to perform a secondfast spin echo sequence so as to acquire diagnostic MR data from each ofsaid diagnostic sub-slabs; and said computer being configured toreconstruct MR image data for each of said diagnostic sub-slabsrespectively obtained from the acquired diagnostic MR data from each ofsaid diagnostic sub-slabs and to emit the reconstructed MR image datafrom the computer in electronic form as a data file.
 8. The apparatus asclaimed in claim 7 wherein said computer is configured to determine theposition of each expanded detection sub-slab relative to theslab-distorting source by calculating, for each respective expandeddetection sub-slab, a slice deformation Δ according to Δ=Δ1−2,${\frac{\Delta 1}{\Delta 2} = {{a\frac{{GS}_{ro}}{{GS}_{ss}}\mspace{14mu}{and}\mspace{14mu} f} = \frac{\Delta 1}{TH}}},$in order to obtain an expansion factor F for each respective expandeddetection sub-slab, wherein$f = \frac{\Delta}{\left( {1 - \frac{{GS}_{ss}}{{aGS}_{ro}}} \right){TH}}$wherein Δ1 is a deformation resulting from excitation of the respectivesub-slabs, Δ2 is a deformation due to said readout gradient, GS_(ro) isan amplitude of the readout gradient applied in the direction of theslice gradient, GS_(SS) is an amplitude of the slice selection gradient,TH is said sub-slab thickness, and a is a coefficient determined in saidcomputer based on TH, a size of an imaging region covered by the appliedreadout gradient, and a resolution of the readout gradient.
 9. Theapparatus as claimed in claim 8 wherein: said computer is configured toset said sub-slab thickness TH of each detection sub-slab, while keepinga total thickness of each detection sub-slab equal, according to theexpansion factor of each detection sub-slab, and adjusting a number ofsaid detection sub-slabs, and the respective positions thereof,according to the adjusted thickness of each detection sub-slab; and whena current number of said detection sub-slabs is equal to said initialnumber, utilizing the position of each detection sub-slab as theposition of each diagnostic sub-slab, and using the expansion factor foreach detection sub-slab as the expansion factor of each diagnosticsub-slab, and when said current number is not equal to said initialnumber, using said current number as the initial number and dividing thecurrent slab into said initial number of detection sub-slabs accordingto said positions, and expanding a thickness of each sub-slab accordingto a predetermined initial expansion factor.
 10. The apparatus asclaimed in claim 7 wherein said computer is configured to expand eachdetection sub-slab symmetrically on opposite sides of the direction ofthe slice selection gradient, or to expand each expanded detectionsub-slab asymmetrically on opposite sides of the direction of the sliceselection gradient.
 11. The apparatus as claimed in claim 7 wherein saidcomputer is configured to operate said MR data acquisition scanner inorder to perform said first fast spin echo sequence as a one-dimensionalfast spin echo sequence or a two-dimensional fast spin echo sequence.12. The apparatus as claimed in claim 7 wherein said computer isconfigured to operate said MR data acquisition scanner in order toperform said second fast spin echo sequence as a one-dimensional fastspin echo sequence or a two-dimensional fast spin echo sequence.